None of what is written below is particularly original, however, I was unable to find a method documented on the internet at the time of writing that successfully created a kickstart driven CentOS 7 USB installer.
My interest was in doing this manually as I require this USB (image) to be created from a script. Therefore, I did not look into using ISO to USB applications - in addition, these typically do not allow custom kickstart files to be used.
Much of the process described below was found on the CentOS Wiki page on Installing from USB key, and from the Softpanorama page on the same subject. I thoroughly recommend reading all of the latter as it highlights the shortcomings/dangers associated with the steps below.